Large scale manufacture of electronic devices such as solar cells is economically feasible only if thin films are used. But in thin film form Silicon and Germanium are amorphous. Presently available amorphous films of pure Silicon or Germanium (Groups IV) fail to behave as good intrinsic semiconductors. The reason is that as these materials are deposited in the laboratory in thin film form their energy band gaps are filled with localized states, generally attributed to microvoids, dangling bonds and lone pairs. Such energy gap states are not removed by doping, annealing or by varying the film deposition conditions. These problems prevent the use of such films in electronic devices especially if they involve changes in the position of the Fermi level, for example by controlled doping with Group III and Group V atoms.
The present invention is based on theoretical observations by Choo and Tong (Proc. VII International Conf. on Amorphous and Liquid Semiconductors, Edinburgh (1977) p. 120-124; Solid State Communications vol. 25, p. 385-387 (1978)) that hydrogen incorporated into amorphous silicon (a-Si) or amorphous germanium (a-Ge) removes the localized states from the gap. Hydrogen acts as a modifier. The hydrogen-modified amorphous silicon or amorphous germanium is a better semiconductor with a well defined energy gap. The function of modification is quite different from that of doping. For a Group IV element such as silicon the doping agents are usually atoms of the nearby Group III and Group V elements. The doping atom introduces an extra free electron from the Group V elements or free hole from the Group III elements thereby greatly affecting the conductivity through the Fermi level. The doping agent does not change the energy gap of the material other than introducing an extra state (donor or acceptor state) in the gap. The main function of a modifier on the other hand is to remove the localized states from the band gap of the imperfect laboratory deposited a-Si or a-Ge films. It does not contribute directly as a free electron or hole carrier, but affects the conductivity through the modification of the energy gap of the matrix amorphous semiconductor. Thus modified, the amorphous Group IV semiconductor is then ready for doping. Another difference between the roles of modifier and dopant can be seen in the amount of modifier and doping atoms required for a laboratory-produced amorphous semiconductor. The amount of modifier needed is directly related to the number of microvoids, dangling bonds, lone pairs and other imperfections in the a-Si or a-Ge which range from 0.1 to 30 atomic percent depending on the process of producing the film. The amount of dopant atoms on the other hand defines the number of current carriers and is usually much less. Modification is the first stage of treatment before the deposited amorphous film can be doped and fabricated into devices such as solar cells, TV vidicons, Xerographic devices, etc. We believe a suitable modifier for amorphous silicon and germanium of the Group IV elements would be Group I atoms like hydrogen and/or Group VII atoms such as fluorine and other halogens.
Several methods have been proposed to introduce modifiers into amorphous silicon and/or amorphous germanium. The better known methods are for example the radio frequency glow discharge of SiH.sub.4 (Spear et al, Solid State Communications, vol. 17, p. 1193-1196 (1975)), SiF.sub.4 or reactive sputtering. In these methods the matrix element silicon is deposited simultaneously with the modifier hydrogen or fluorine. This approach suffers from difficulties of controlling the amount of modifier atoms introduced in the sample. The presence of modifier atoms during deposition also affects the structure of the film. The result is that the product is unstable under heat and light illumination and has poor reproducibility. The hydrogen content of the resulting sample is too high (about 5 to 30 atomic percent) for the hydrogen to be considered as a modifier. From the principles of the present invention one can see why the above approach does not meet with greater success. Based on the general principles outlined above, one should first prepare a film of the matrix semiconductor of Group IV element which contains as few gap states as possible, followed by the introduction of just the sufficient amount of modifier atoms to remove the gap states.